Electrophotography is a useful process for printing images on a receiver (or “imaging substrate”), such as a piece or sheet of paper or another planar medium, glass, fabric, metal, or other objects as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (i.e., a “latent image”).
After the latent image is formed, charged toner particles are brought into the vicinity of the photoreceptor and are attracted to the latent image to develop the latent image into a visible image. Note that the “visible image” may little or no visibility to the naked eye depending on the composition of the toner particles (e.g., clear toner).
After the latent image is developed into a visible image on the photoreceptor, a suitable receiver is brought into juxtaposition with the visible image. A suitable electric field is applied to transfer the toner particles of the visible image to the receiver to form the desired print image on the receiver. The imaging process is typically repeated many times with reusable photoreceptors.
The magnitude of electric field to be applied to transfer an appropriate amount of toner depends on a variety of factors. Examples of such factors include the resistance of a transfer member, the toner charge-to-mass, the ambient environment (temperature and relative humidity or absolute moisture content), physical and electrical properties of a receiver (thickness, width, resistivity), the machine printing speed, and the side of the receiver being printed upon.
A variety of methods have been developed in the art for setting image formation parameters, including a transfer bias level, in response to measurements made of various imaging system attributes.
U.S. Pat. No. 5,963,756 to Sakai et al., entitled “Color correction in a color image formation apparatus,” discloses the formation and measurement of monochrome and multicolor patch densities and adjusting color image formation process parameters based upon these measurements. The measured patch density values are compared to target values and the differences between the measured and target values are used to change the image formation condition.
U.S. Pat. No. 6,477,339 to Yano et al., entitled “Image forming apparatus with current detector and voltage control based on detection result,” describes measuring the resistance of a transfer belt by applying a constant current and measuring the voltage required to sustain that current.
U.S. Pat. No. 6,985,678 to Maebashi et al., entitled “Color image forming apparatus and control method therefor,” discloses the formation and measurement of monochrome and multicolor patch densities and adjusting color image formation process parameters based upon these measurements. The measured patch density values are compared to target values and the differences between the measured and target values are used to change the image formation condition.
U.S. Pat. No. 7,151,902 to Rakov et al., entitled “Toner transfer technique,” discloses an algorithm that varies the transfer bias current depending upon a control voltage value that is linearly proportional to the toner charge-to-mass raito. A potential drawback with this approach is that the control voltage value may vary due to factors other than toner charge-to-mass, resulting in an erroneous adjustment to the transfer bias current.
U.S. Pat. No. 7,340,191 to Yamada, entitled “Image forming apparatus featuring variably-controlling a primary-transferring condition based on a detection result of combined toner images,” discloses a method for controlling a bias current in an electrostatic transfer system. The method includes measuring the density of a single color patch and a multi-color patch. If the difference between the measured densities exceeds a predefined threshold, the bias current is reduced.
U.S. Pat. No. 7,450,871 to Yamada, entitled “Image forming apparatus with an adjustment function for adjusting color taste of toner image,” discloses forming mixed toner images on a transferring medium, detecting the mixed toner images, and controlling a transferring condition based on the detection result to adjust a color preference.
U.S. Pat. No. 8,019,246 to Yamada, entitled “Image forming apparatus,” discloses the adjustment of a transfer bias level based upon the measurement of the resistance of a transfer member in combination with a measurement of the absolute moisture content.
U.S. Pat. No. 8,687,989, entitled “Transfer unit with compensation for variation,” by Zaretsky discloses the use of a charger to estimate the variation in electrical properties of a static dissipative member and controllably adjust the electric transfer field to compensate for the estimated variation.
U.S. Pat. No. 8,737,854, entitled “Printing system with receiver capacitance estimation,” by Zaretsky discloses the use of a charger and non-contact voltmeter to estimate the capacitance of a receiver and subsequently adjust the transfer power source to produce an electrostatic transfer field commensurate with the estimated capacitance.
However, none of the above prior art characterize the transfer performance using in-situ measurements of monochrome and multi-chrome patches, and determine an optimal transfer bias level based upon this characterization.
There remains a need for an improved method to determine transfer bias settings in an electrophotographic printing system having a plurality of printing modules.